1. Field of the Invention
The present invention, in its most general aspect, relates to a method for measuring characteristic properties of a pneumatic tyre for vehicle wheels. More specifically, the invention relates to a method for detecting the size of the footprint area of a pneumatic tyre for vehicle wheels in high-speed motion conditions, and the distribution of pressure on such a footprint area.
2. Description of the Related Art
Throughout the present description and the subsequent claims, the term: characteristic properties of a pneumatic tyre, is used to indicate, for example, the dimensions and/or the shape of the footprint area of the tyre, the distribution of pressure (or vertical forces) acting locally on the footprint area of the tyre, the tyre inflation pressure, the distribution of longitudinal and/or lateral forces acting on the tyre and the rolling resistance.
The term: footprint area, is used to indicate that zone of the tyre wherein the exchange of forces between the tyre and the road surface (or between the tyre and a device simulating the road surface, such as a rolling runway of a drum simulating the road surface) takes place, i.e. that zone of the tyre which is in contact with the road surface (or with the device simulating the road surface).
The term: high-speed, is used to indicate a speed greater than 30 km/h, preferably greater than 100 km/h, even more preferably greater than 200 km/h.
As known, a considerable part of the research in the field of tyres for vehicle wheels has as a primary object that of optimizing the design parameters of the tyre to ensure adequate behavior thereof in the foreseen operating conditions. For such a purpose, tyres are inspected and tested to detect some properties representing the behavior of the tyre in operation.
For example, it is used to measure the size and/or the shape of the footprint area of a tyre and to detect the variation of such a size and/or shape as the rolling speed of the tyre varies. Indeed, it is known that a properly designed tyre has a wide, rectangular or square footprint area which substantially does not change as the rolling speed of the tyre changes.
Also, it is used to measure the distribution of pressure on the footprint area of the tyre and, even in this case, to detect the variation of such a distribution of pressure as the rolling speed of the tyre varies. Indeed, it is known that a properly designed tyre has a substantially uniform distribution of pressure between the shoulders and the center of the tyre, such a distribution being also substantially invariable as the rolling speed of the tyre varies.
Thus, for example, U.S. Pat. No. 5,749,984 discloses a method for monitoring a tyre wherein a measurement of the size of the footprint area of the tyre is carried out by a sensor suitably housed inside the tyre. In the preferred embodiment thereof, the sensor is a piezoelectric element which deforms when the tyre comes into contact with the ground thus generating an electrical signal proportional to the speed of deformation thereof. The position of the sensor inside the tyre is described as being particularly critical for generating a suitable electrical signal.
The Applicant has observed that this system is an invasive detection system (i.e. the sensor is provided inside the tyre) and thus it involves complication in the production process of the tyre, as well as it requires the production of tyres ad hoc. The Applicant has also verified that a system as the one described in the patent identified above does not allow the actual duration of contact between tyre and road, and therefore the length of the footprint area of the tyre, to be determined with precision. Indeed, the signal generated by the sensor is influenced by phenomena which take place before or after the tyre contacts the road. The Applicant has also observed that such a patent does not provide any indication of the shape of the tyre footprint, nor of the distribution of pressure on the footprint area.
A non-invasive system for detecting the distribution of pressure on the footprint area of a tyre under almost static conditions is disclosed in patent application EP 0 656 269. Such a system provides for the use of a plurality of arrays of load sensors directly applied on the road surface. Such sensors directly measure the vertical forces acting upon the footprint area of the tyre and generate an electrical signal proportional to such forces. The Applicant observes that, in line with the disclosure of the aforementioned document, such a system is capable of detecting the distribution of pressure for static conditions or for rolling speeds of the tyre lower than 5–6 Km/h, greater speeds being ruled out due to the limited and/or insufficient frequency response of the load sensors.